1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and tools used to measure formation porosity in oil and gas industry, more particularly, to methods and tools using microstructured semiconductor neutron detectors.
2. Discussion of the Background
In the oil and gas industry, formation porosity is measured to identify oil and gas reserves. Although other techniques may be employed to determine formation porosity (e.g., sonic and Nuclear Magnetic Resonance), the porosity measurements using neutrons is the most frequent.
Down-hole neutron-porosity tools may be wireline or logging (or measuring) while drilling (LWD/MWD). The principal difference between LWD and wireline tools is the service environment. LWD tools operate during the drilling process and are subjected to the high levels of vibration and shock generated by drilling through rock. Wireline tools are conveyed in and out of the borehole on a cable after drilling, and, therefore, do not experience shock and vibration. In both cases, the tool operates at temperatures as high as 175° C. sometimes higher.
As illustrated in FIG. 1, down-hole porosity measurements are performed using a neutron source 10 and two detectors or arrays of detectors, a “near” neutron detector 20 and a “far” neutron detector 30, which are located at different distances from the neutron source 10. The neutron source 10 and the neutron detectors 20 and 30 are usually encapsulated in a chassis 40. The chassis 40 is lowered in a borehole 50 that penetrates a soil formation 60. Some of the neutrons emitted by the neutron source 10 towards the soil formation 60, loose energy (i.e., are “thermalized”) and are deflected towards the neutron detectors 20 and 30 due to collisions or interactions with nuclei in the formation 60.
The detectors 20 and 30 detect some (depending on each detector's efficiency) of these neutrons with lower (thermal) energy. The ratio of the counting rates (i.e., number of detected neutrons/time) in the two detectors 20 and 30 is directly related to the porosity of the formation 60.
The probability of an interaction of a neutron and a nucleus (i.e., a nuclear reaction) can be described by a cross-section of the interaction (i.e., reaction). A detector's efficiency is proportional with the probability of an interaction occurring when a neutron enters the detector's volume. The neutron detectors are built based on the large probability (i.e., cross-section) of a thermal neutron being captured (i.e., interact/react) with three nuclei: helium (3He), lithium (6Li) and boron (10B). Other particles such as, the α particle (24α) and the proton (11p) result from the reaction of the thermal neutron with these elements. A calculable amount of energy (Q) is emitted as a result of the neutron capture reaction. This emitted energy may be kinetic energy of the resulting particles or gamma rays. The energy is dissipated by ionization, that is, formation of pairs of electron and positively charged particle. These pairs can be collected, for example, in an electrical field, and, thus, generate a signal recognizable as a signature of the neutron capture reaction. The larger is the emitted energy, the larger is the amplitude of the signature signal.
Some other particles (e.g., gamma rays) besides the targeted neutrons may cross the detector simultaneously. A good detector should exhibit characteristics that would allow discrimination between capture of a thermal neutron and other untargeted nuclear reactions that may occur. To facilitate discrimination between a neutron capture reaction and a gamma ray, the energy emitted in the neutron capture reaction (Q) should be as high as possible.
The three most common neutron capture reactions used for neutron detection are illustrated in Table 1:
TABLE 1Thermal neutron cross NameReactionQ (MeV)section (barns)10B(n, α)510B + 01n → 37Li + 24αGround 2.7923840Excited 2.316Li(n , α)36Li + 01n → 13H + 24α4.78 9403He(n, p)23He + 01n → 13H + 11p0.7645330
In the above table, relative to the 10B(n,α) reaction “Ground” means that the resulting 37Li is in a ground state and “Excited” means that the resulting 37Li is in the first excited state.
Traditionally, detectors based on 3He(n,p) reaction have been used in neutron porosity measurements performed in the oil and gas industry, due to their relatively low cost, ruggedness, good detection efficiency, and insensitivity to gamma rays (i.e., the cross section for an interaction of the gamma ray with 3He is very small). The detection efficiency of these 3He based detectors can be improved by using higher pressures of the 3He gas, but the use of higher pressures results in increasing the cost of the detectors and of the high voltage required to operate them, which adversely affects the associated detector electronics. Additionally, the critical worldwide shortage of 3He makes it necessary to develop alternate neutron detectors for neutron porosity measurements in the oil and gas industry.
Lithium-glass scintillation detectors are currently used in some logging tools. The detection efficiency of the detectors based on 6Li(n,α) reaction depends on the amount of 6Li in the detector material. A common lithium-glass used for down-hole logging is GS20, which has an isotopic ratio of 95% 6Li and a total lithium composition of 6.6%. Although the cross section for an interaction of the gamma ray with 6Li is significant, the large amount of energy (Q) resulting from the 6Li(n,α) reaction enables a reasonable discrimination from reactions induced by gamma rays. However, the poor energy resolution of lithium-glass detectors at room temperature diminishes further at temperatures as low as 150° C., rendering their use limited to relatively shallow wells. In the lithium-glass scintillation detectors, the lithium-glass is coupled to a photomultiplier tube (PMT) that introduces electronic noise at elevated temperatures and is mechanically fragile.
Accordingly, it would be desirable to provide neutron detectors having a good detection efficiency (i.e., large cross section for neutron capture), good discrimination relative to gamma rays, and can be used in the logging shock and vibration environment (e.g., during drilling) and at high temperatures (e.g., over 175° C.).